The present invention relates to a process for producing polyester polyols with secondary hydroxyl end groups, including the step of the reaction of a polyester including carboxyl end groups with an epoxide. It further relates to polyester polyols with secondary hydroxyl end groups that are obtainable by this process and to a polyurethane polymer that is obtainable from the reaction of a polyisocyanate with such polyester polyols.
Technically relevant polyester polyols for the production of polyurethane polymers exhibit primary hydroxyl end groups as a consequence of the α,ω-diols that are used for their synthesis. The use of diols with wholly or partly secondary hydroxyl end groups—such as, for instance, 1,2-propylene glycol or dipropylene glycol—results in polyester polyols that with respect to the end groups are, to some extent, endowed just like the diols synthesising them. In the case of 1,2-propylene glycol, about 50% of the hydroxyl end groups would be secondary.
Diols that exhibit only secondary hydroxyl end groups—such as, for example, 2,3-butanediol—play no role on a technical scale by reason of the commercially available quantities and the cost. An additional aggravating factor in the case of all diols exhibiting secondary hydroxyl groups in the synthesis of polyester is that the rate of conversion with dicarboxylic acids is lower.
Particularly disadvantageous, furthermore, is the fact that, as a consequence of the numerous short alkyl side groups, the properties of the polyurethanes produced from polyesters of such a type are distinctly poorer than those of polyurethanes that are obtained from α,ω-diols. Accordingly, conventional polyester polyols, which are produced with the aforementioned diols with at least partly secondary hydroxyl end groups, are both more expensive in the manufacturing costs, in part more expensive in the material costs, and also less suitable for producing high-quality polyurethanes. For this reason, up until now polyester polyols with secondary hydroxyl end groups have, in contrast to polyether polyols, had no relevant significance technically.
It would be desirable to have polyester polyols available that, in their interior, contain α,ω-diol units and, at their chain end, a unit with secondary hydroxyl groups. A structure of such a type would have the consequence of a diminished reactivity towards polyisocyanates and makes it possible, for example in the field of the polyurethane flexible foams, to employ, besides the amine catalysts which mainly drive the water reaction, also additional urethanisation catalysts such as tin salts. In particular, this opens up the possibility, widely utilised in the field of the polyether polyurethane foams, of matching these two reactions better to one another and thereby, for example, of obtaining processing advantages in the production of polyester polyurethane flexible foams.
The functionalisation of carboxyl groups in the course of polyester-polyol synthesis is disclosed in DE 36 13 875 A1. For the purpose of producing polyester polyols with an acid value of less than 1, with a hydroxyl value from approximately 20 to approximately 400, and with a functionality of, expediently, 2 to 3, polycarboxylic acids and/or the anhydrides thereof and polyhydric alcohols are condensed. This happens advantageously in the absence of customary esterification catalysts at temperatures from 150° C. to 250° C. and optionally under reduced pressure. Polycondensation is effected as far as an acid value from 20 to 5, and the polycondensates obtained are then alkoxylated per carboxyl group with 1 mol to 5 mol alkylene oxide, for example 1,2-propylene oxide and/or preferentially ethylene oxide, in the presence of a tertiary amine. The tertiary amine is selected from the group comprising N-methylimidazole, diazabicyclo-[2,2,2]octane, diazabicyclo[5,4,0]undec-7-ene and pentamethyldiethylenetriamine. The catalyst is expediently employed in a quantity from 0.001 wt. % to 1.0 wt. %, relative to the weight of the polycondensate. Advantageously, alkoxylation is effected at temperatures from 100° C. to 170° C. and under a pressure from 1 bar to 10 bar.
In the process according to DE 36 13 875 A1 the esterification mixture is polycondensed as far as an acid value from 20 to 5. It is stated as essential that the melt condensation is not terminated too early. If, for example, alkoxylation is effected at an acid value of 25 or greater, the water content of the esterification mixture is said to be excessively high. This would, however, result in undesirable side reactions. If the synthesis of the polyesters is terminated at an acid value from 20 to 5, this means that a comparatively high proportion of terminal hydroxyl groups originating from the alcohol component, and therefore, as a rule, of primary hydroxyl groups, is already present. For the purpose of shortening the synthesis-time, the residual carboxyl groups are then converted with epoxides, whereby terminal hydroxyl groups originating from the epoxides are obtained.
EP 0 010 804 A1 discloses a powder coating on the basis of carboxyl-group-terminated polyesters, an epoxy compound and a choline compound of the formula [Y—CH2—CH2—N—(—CH3)+n Xn−, in which X is OR or —O—C(O)—R and R is hydrogen or a C1-40 group and Xn− is an anion. Preferentially Y is OH or a —O—C(O)—R group. These powder coatings are less susceptible to yellowing and are not toxic. However, according to this document the epoxy compound exhibits, on average, two or more epoxy groups per molecule. The epoxy compound serves here in order to cross-link polyester molecules with one another, and not for synthesising OH-terminated polyester molecules.
DE 28 49 549 A1 discloses a process for producing polyether polyester polyols by conversion of a polyether polyol with a polycarboxylic acid anhydride to form an acid half-ester. Subsequently the acid-half-ester is converted with an alkylene oxide into a product with an acid value of less than 5 mg KOH/g. The conversion of the alkylene oxide with the acid-half-ester is carried out in the presence of 50 ppm to 100 ppm, relative to the initial polyether polyol, of a trialkylamine with 2 to 4 carbon atoms in the alkyl chain. The polyol that is obtained, however, is still based on polyethers and not on polyesters.
U.S. Pat. No. 4,144,395 discloses a process for producing polyether ester, wherein by conversion of a polyether polyol with anydride a half-ester is formed which is converted with epoxides into polyether ester, whereby alkylamines are employed as catalysts. The half-ester obtained as intermediate in Examples 1 and 2 of U.S. Pat. No. 4,144,395, formed from maleic acid (0.75 mol) and trifunctional polyether polyol (0.75 mol), differs structurally from the polyester including carboxyl end groups that is employed in accordance with the present invention.
Consequently a demand continues to exist for alternative production processes for polyester polyols with secondary hydroxy end groups.